The purpose of this study is to further understand the pharmacology of analgesia to intractable pain states in the human which will hopefully spur drug development in this area. This Phase I trial is to determine the safety of acute intrathecal bolus administration of adenosine through the anticipated therapeutic range (dose-ranging study). Adenosine is a naturally occurring neurotransmitter known to be released spinally to produce analgesia. Adenosine and adenosine agonists are highly effective in animal models of neuropathic and inflammatory pain against which standard analgesics are often ineffective. A similar formulation of adenosine which is marketed in Sweden has undergone preclinical toxicity screening, Phase I trials, and is in early Phase II study by a physician investigator there, and initial results show efficacy in patients with neuropathic pain. In this Phase I trial of spinal administration of adenosine we will study 25 healthy volunteers. To determine whether adenosine produces analgesia, acute thermal stimulation will be performed on the leg and arm, using the random thermal stimuli (38 and 510C), asking the volunteer to rate the pain of each stimulus, using his pain magnitude scoring system.